From automotive technology, vehicles with a hybrid drive and with hydraulic transmission control are known. In such drive-trains a primary transmission pump is usually provided for supplying pressure medium. However, when the internal combustion engine is switched off and the drive is purely electric, the primary pump cannot build up the necessary pressure to start because from rest the electric machine requires maximum torque for starting. Accordingly it is known to use an auxiliary electric pump, which provides the shifting elements with the pressure needed for torque transmission.
During starting processes for example on an uphill gradient, the vehicle can roll backward. However, when rolling backward the primary pump, which is connected to the transmission input shaft, rotates in the wrong direction and so draws pressure medium out of the transmission control system and delivers it back into the pressure medium sump. In this situation even the auxiliary electric pump can no longer supply enough oil since the quantity returned by the primary pump is too large. Consequently, the system pressure in the transmission control system collapses and the shifting elements can no longer transmit torque, so they operate in a slipping mode. This uncontrolled slipping operation increases the stress on the shifting elements and results in oscillations in the drive-train and in undesired bucking of the vehicle.
In the case of hybrid vehicles, rolling of the vehicle backwards can be counteracted with the help of the electric machine. However, it should be noted that at rest, in the case of synchronous machines, the rotating field is static and the power electronics system is therefore exposed to high current loads. With synchronous machines, it is true that this effect is somewhat less pronounced since even at rest a rotating field is produced by virtue of the slipping. However, this electrical prevention of rolling back by maximum torque is subject to severe time limitation. Furthermore, the use of the short-circuit torque at very low rotational speeds is critical since this can fall abruptly and the vehicle then moves backward without any braking.